The invention relates to gauging a liquid by means of a piezoelectric pellet.
More particularly, the invention relates to a liquid gauging device using a piezoelectric pellet in a housing.
Document EP 0 182 140 discloses devices for gauging liquid depth by means of piezoelectric pellets.
In that technique, the piezoelectric cells are adapted to transmit a sound wave from the bottom of a tank containing a liquid and to detect said wave, e.g. after reflection at the surface of the liquid, in order to determine the depth of the liquid through which the wave has traveled on the basis of the time interval between transmission and reception.
Document U.S. Pat. No. 5,095,748 describes in particular a gauging system having targets at known depths to which the sound waves are directed, said targets serving as reference depths for subsequently determining the depth of the liquid.
Unfortunately, those prior techniques present certain drawbacks.
The reference sound waves directed towards the targets and the sound waves for measuring the surface are sometimes confused, giving rise to false measurements or to measurements mixed with interference.
The invention seeks to mitigate those drawbacks.
To this end, the invention provides a piezoelectric liquid gauge system in particular for a motor vehicle fuel tank, the system comprising piezoelectric means suitable for transmitting a sound wave in a liquid and for detecting said wave in order to determine the distance traveled by the wave in the liquid on the basis of the time between transmission and reception, said piezoelectric means comprising two distinct active piezoelectric portions suitable respectively for directing a sound wave towards a reference obstacle and a sound wave towards the surface of the liquid to be gauged, wherein the pellet is contained in a housing suitable for being fixed in position in the tank in which measurement is to be performed, the reference obstacle extending over a face of the housing which faces one of the active portions, said face also having a measurement opening placed facing the other active portion and suitable for allowing the measurement sound waves to pass through.
The invention advantageously further comprises the following characteristics taken singly or in any technically feasible combination:
both active portions come from a common piezoelectric pellet;
the piezoelectric pellet is substantially disk-shaped, the active portion suitable for directing a sound wave towards the liquid surface being situated in the center of the disk and having a circular outline, the active portion suitable for directing a sound wave towards the reference obstacle extending around the central portion;
the opening in the wall of the housing facing the measurement active portion is of substantially the same dimensions as said measurement portion, said dimensions of the opening nevertheless being slightly greater;
the system includes a thermistor connected in parallel with the piezoelectric measurement means, said thermistor presenting resistance that is lower than the resistance of the piezoelectric measurement means throughout the operating temperature range of the vehicle;
the system includes a diode;
the system includes calculator means enabling the depth of liquid in the tank to be determined on the basis of the time separating transmission and reception of sound waves by the two active portions;
the calculator means are also suitable for determining the temperature of the liquid in the tank by measuring the resistance across the terminals of the thermistor;
the system includes support means for fixing the housing in position at the bottom of the tank;
the housing has means suitable for holding the piezoelectric means in position inside the housing;
the housing includes means suitable for holding the electrical connections of the pellet in position inside the housing;
the reference obstacles comprise wave reflectors, e.g. oriented at 45xc2x0 relative to a vertical direction of the device placed in the tank and of bottom surface facing towards the nearest side walls of the housing;
the area of the reference obstacles is substantially equal to the area of the measurement opening; and
the inside of the housing is subdivided into two volumes: a reference first volume; and a measurement second volume; the two volumes being separated by means of a separator tube extending from the top wall of the housing substantially in register with the separation between the two active portions of the pellet, and terminating before making contact with the active pellet fixed to the bottom of the housing.